Topic: motorcyle dosen't go over 3000 rpm

[ofer]

Hi all, I am new here just bought a 1982 cb650. the motorcyle dosen't go over 3000 rpm. When I accelerate it starts to lose power when it gets to 3000 rpm and it starts to backfire. I cleened the carburators and checked the float height. But still no cahnge.
can you please help- what should I do?
Thanks

[wallym]

One simple check would be to open the fuel cap after a run, is there a whosh of air when its open, If tank vent is blocked no air can get in and it stops fuel getting out to the carbs.

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[calj737]

Is it running pods instead of the stock airbox? 650s don't tolerate swaps like that very happily.

[ofer]

no pods. runs with original air box.

[calj737]

Well accelerating and hitting a limit in RPMs followed by profuse backfires is a pretty strong indication of fuel starvation. It could be clogged up main jets on the carbs, it could be (as posted) vent blockage. It could be a dozen small things.

Can you provide any better detail on the history of the bike? I know you just bought it, but was it a daily rider? Had it been sitting? Did you test ride the bike beforehand?

Use a PhotoBucket account or similar and post some pictures with some detail of the bike, from sides of the carbs, etc.

[b52bombardier1]

When it hits that 3000 rpm limit, reach down and give the carbs just a little choke.  Does it run better??  If so, it's a classic sign of a dirty carburetor as advised above.

  You need to get more aggressive with your cleaning. 

Rick

[ofer]

Thanks guys for the fast reply.
I took a ride now with the gas tank cap open and it didn't help. also tried to lift the choke a few times when it got to 3000 rpm and it did not help the bike, One time he choked and almost stopped running.

I don't know any thing about the bike. I bought it online. The seller told me about the problem and I thought a carb cleaning will solve the problem just as you thought. When I got the bike I cleaned the carbs really good with carb spray cleaner thoroughly.
The carb boots are new and dont have any cracks. I removed the vacume valve and conected the gas line from the gas tank directly to the carb. I opened the pilot screw 2.5 turns out from close position.

Can you think of another thing that makes the problem?
Thanks
Ofer

[BomberMann650]

Did you inspect the rubber connectors when the carbs were off for cleaning?

And are the clamps on TIGHT now that they were reinstalled?

[flatlander]

how is the throttle operation on the carbs?

actually, the first thing should be to thoroughly go over the bike and get everything in good tune. that gives you a starting point as now you don't really know anything of it's state so you'll be just chasing your tail. start with the 3k miles maintenance procedure described in the honda shop manual.

[ofer]

I looked at the  rubber connectors when the carbs were off the bike but i did not took them off the engin.
The clams are tight.
I will do the 3K maintenance and report back
Thanks

[carnivorous chicken]

Start with the 3k miles maintenance procedure described in the honda shop manual.

This, to give you a baseline. And while timing it, check that your spark advance is working properly.

[b52bombardier1]

+1 on checking the timing and spark advance.  Does the bike idle well - at a thousand RPM or so?

Rick

[madmtnmotors]

Include a compression check while you're at it.

[tlbranth]

Agree with the basic tuneup recommendation. It sounds like fuel starvation or retarded timing but best to just give it the Full Monty and see how it goes. Since choking makes it worse, you might try running it without the air filter to see if it improves any. I innocently installed an Emgo air filter when resurrecting my 750 and it seriously affected its running. Plug condition should indicate a rich or lean condition. Timing is easy to do (check that advancer functions).

[grcamna2]

The fuel valve on the CB650 can be a little tricky to have it work 100%,keep checking the amount of fuel flowing to your rack of carbs.,a clear fuel hose on the carbs. might help you to see if the fuel is flowing.The spark plugs are probably very dirty by now? how do they look ?

[RAFster122s]

The CV carbs you have are notorious for clogged tiny passages inside.  Just spraying them out with carb cleaner will not do much. An ultrasonic cleaning with the Yamaha carb cleaner or the Home Depot purple Simple Green @50% cleaner to distilled water can get into the jets and passages.  The ACV ( Air Cutoff Valves) on the sides of the carb body need pulled and covers removed and hopefully all your jets come out by screwdriver. The pressed in versions are difficult to remove. Remove all the brass jets.  Ultrasonic clean the carbs until they no longer change the solution color. Pull the plastic pieces out on top of the carb body as well.

Check the tiny passages by spraying through them with a straw attachment on the carb cleaner can...should flow freely a good stream.

Drop the jets in a small container of CLR for about a minute or so.  They should stop bubbling as much by then. Rinse and then polish up the outsides and rinse/spray them off with carb cleaner. The emulsion tubes have small holes in them that often get clogged because they are the smallest port/passage on the bike's carb if I recall correctly.
Your problem is leaning running because of the backfires in acceleration.  If it were only popping/backfire burble on decelleration you would have bad acv valves.  I doubt it is your emulsion jets but then need a through cleaning to give good operation.  The emulsion tubes help vaporize the fuel and serve to lean out the mixture, but they help ensure proper operation.
Georgefix on ebay carries several good parts for the CX bikes.
ONLY use Keilhn carb brass parts.  Check yours for the proper K logo on them.
I like to use a numbered drill to ensure those jets are clean. Do not remove brass, only clean out other deposits, not making those openings Any larger.
Reassemble

[PlungerWaffle]

I have had similar power isues on my 1980 650 custom. Although i have PD i had to clean the snot out of them things before i could get em working. Cleaned em 15-20 times, just blasting them might not work, pull everything apart that comes apart and clean things out with a paper clip or a sewing pin. Now im also wondering if maybe timing could be off? Or perhaps your plugs arent plugged into the right coils? Just an idea. Just a thought though. Good luck!

[RAFster122s]

The CV carbs have a bad reputation for getting very finnicky.The effort to get all the tiny passages clean is the reason for this. They really need a good ultrasonic cleaning with a good cleaning solution so the hard white deposits they accumulate open up. The CX and GL 500/650 bikes along with a bunch of early 80 s bikes and even the Goldwings used these carbs.  Larry Cargill over on CX/GL forum is on par with Mike Nixon's skills when it comes to cleaning these.  Larry developed a procedure that works great on the carbs. I have his book on the process.  Odds are the numbered drills for your bike are different than the CX and GL 500s. I followed Larry's procedure and they came out great and it runs so much better.
Bench and vac sync help a lot also.  Mike Nixon charges over a grad to do all 4 and I think Larry charges over 300 for a pair. It is an involved process.  Larry now vapor blasts the carb bodies and polishes the aluminum to very high gloss. Gets the external clean before vapor blasting and then ultrasonic cleaner for 1/2 hour to hour in his industrial ultrasonic machine. Ensures all the ports are flowing freely and cleans the brass and chases the holes in the jets with numbered drills to get any crud out of the jets.  You do NOT want to see any brass shavings in the material that comes out of the jets.  Even running them through CLR and ultrasonic I still had a little build up in the jets...not much but with these tiny holes it makes a difference. Result is a bike that idles silky smooth and is responsive as it should be ( within limitations of bike).

[RAFster122s]

The 650 is also know for ignitor issues with its CDI boxes as well.  So, if your carb problem isn't electrical it is electrical.  90% of carb problems turn out to be electrical when you have clean carbs. 
If a air intake boot has swelled it may not be possible to clamp it tight enough to be leak free.  WD40 or ether sprayed at the joints between boots and carbs or head will tell you if you have a leak.  Orings are often used on intake manifolds to the head.  These can leak as well when bolted up tight.  They will be hard square brittle lumps in the grooves of the manifold if they need replaced.

Check you spark plug caps resistance by removing from wire end (screw off) and measure ohms...should be around 5k if like the other cb bikes. I f more than 500 ohms off that figure replace them and cut the wire ends back about 3/4" for fresh connection.
David

[scunny]

the goo just oozes out of the pulse thingy's, just seal them back up with silicon or something of your choice, they either work or don't, no halfway. mine have been like that for 5 or 6 years. my pick is the carb diaphram  or the advancer not being free to rotate. 

[John Eberly]

Ofer,

One more simple thing nobody mentioned - try the bike without the air filter element. Your filters could be plugged and starving the engine for air.

[ofer]

Thanks guys,
I did a full 4K miles maintenance and tune up.
I found auto that the problem is with the piston rings. The compression was around 100 PSI  for all cylinders and after purring a tea spun of oil into the cylinder the compression went up to 140 PSI so my conclusion is that I need to replace the piston rings.

Do you think that this was the problem?

Ofer

[DaveBarbier]

I don't think so.

You checked the advance mechanism? I would check the timing with a strobe light and see if you're getting an erratic fire when the issue occurs.

I'm assuming this problem always happens regardless if you're moving or not?

This petcock has a vacuum switch, right? Have a good flow if you remove the line from the carbs and drain it into a container? You'll have to apply vacuum to the correct line to allow the petcock to flow.

[Flatlander80]

+1 to checking the advance mechanism... I have a 1980 CB650 and that was a problem. You have to pull the whole timing mechanism out from inside the front right cover, and make sure the two weights move freely. That cover isn't water tight, so if the bike was stored outside for a long time, there can be quite a bit of rust/corrosion in there... 3000 rpm is right around the magic number where those weights should start to fly out and advance the timing. If they're stuck, the engine just bogs...


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[ofer]

I checked the advance mechanism it is free and clean. I checked the timing, did a dynamic test with gun light and all is fine including the advance over 2750 RPM.
The bike idle nicely except some smoke from the right side exhaust. When it stand the RPM go over 3000 when I open the throttle but not as smooth as I expect. The problem is when I ride he is losing its power over 3000 RPM.
I canceled the vacuum petcock and connected the gas tank directly to the carb.